Ultra-high molecular weight polyethylene fiber is the third generation high performance fiber emerged in the early 1990s. The relative molecular weight of ultra-high molecular weight polyethylene fiber is over 1 million, the molecular shape thereof is linear straight chain structure, and the degree of orientation thereof approaches 100%. Due to excellent mechanical properties and outstanding comprehensive performances of the ultra-high molecular weight polyethylene fibers, the application potential thereof is huge. Ultra-high molecular weight polyethylene fibers are applicable to both military use and civil use, and are widely used in various aspects, such as bulletproof products, explosion proof equipment, high strength cable and sports equipment, etc.
Ultra-high molecular weight polyethylene fibers are made by processing ultra-high molecular weight polyethylene resin. At present, ultra-high molecular weight polyethylene resin is mainly prepared by Ziegler low-pressure slurry process, i.e., the process of polymerizing ethylene at certain temperature and pressure and producing products with different molecular weights, using β-TiCl3/Al(C2H5)2Cl or TiCl4/Al(C2H5)2Cl as main catalyst, triethyl aluminum as co-catalyst, and saturated hydrocarbon at the temperature of 60 to 120° C. as dispersion medium.
Patent literature CN1033703 provides a method for preparing ultra-high molecular weight polyethylene, which can regulate the molecular weight of the ultra-high molecular weight polyethylene. According to the patent, ultra-high molecular weight polyethylene resin having good particle-size distribution and molecular weight that can be adjusted within a range from 0.6 to 6.1 million can be prepared under the temperature of 65 to 85° C., using MgCl2 and ZnCl2 composite supported titanium catalyst. According to this invention, the molecular weight of the product can be adjusted by regulating the Zn/Ti molar ratio. The catalyst used therein is a composite supported catalyst. The resin prepared through polymerization according to this patent is not used in fibers. The resin particle has smooth surface, and comprises no porous and gyms-like structure.
Patent literature CN1999809336 relates to a method for producing high-strength polyethylene fibers, wherein a polymerization mixture comprising from 99 to 50 parts by weight of (A) and from 1 to 50 parts by weight of (B), where (A) is high molecular weight polymer comprising mainly ethylene component and having a weight average molecular weight to number average molecular weight ratio (Mw/Mn) of no greater than 4 and an intrinsic viscosity η of no less than 5 dl/g, and (B) is an ultrahigh molecular weight polymer having an intrinsic viscosity at least 1.2 times that of high molecular weight polymer (A), is dissolved in solvent to a concentration of from 5% by weight to 80% by weight, then spun and drawn. The method according to this patent can efficiently manufacture high-strength polyethylene fibers having an intrinsic viscosity η of no less than 5 dl/g, a strength of no less than 20 g/d and an elasticity modulus of no less than 500 g/d. According to this patent, a mixture of two ethylene polymers having different intrinsic viscosities is used. For fibers made from the mixture, uniformity and stable performance can hardly be achieved, and the mechanical performances thereof are lower than those of ultra-high molecular weight polyethylene fibers.
Patent literature EP277750 provides an ultra-high molecular weight polyethylene resin for producing ultra-high strength fibers, having an intrinsic viscosity 11 in a range of 5 to 30 dl/g and a particle size in a range of 1 to 300 μm. Patent literature JP 2007297763 provides a method for producing high-strength polyethylene fibers, using high molecular weight polyethylene resin having an intrinsic viscosity greater than 8 dl/g. Patent literature CN 101421444 provides a spinning method for high strength polyethylene fibers that are difficult to obtain by conventional gel spinning methods. The high molecular weight polyethylene resin according to this patent has an intrinsic viscosity greater than 8 dl/g. When the polymer dope for spinning is prepared, ethanol compound, in which the resin is insoluble, is added. The mixture of decalin, poor solvent and ethanol compound is used as solvent, in which the polymer swells and dissolves. Thus, after spinning, high strength fibers can be obtained. Currently, during the production of fibers, ultra-high molecular weight polyethylene resin with relatively low intrinsic viscosity, i.e., relatively low molecular weight, is mainly used, which would significantly reduce the mechanical properties of the final fiber product.
At present, gel spinning process is mainly used for the production of ultra-high molecular weight polyethylene fibers. Gel spinning process requires the use of a specific solvent before the melt spinning and drawing, so that the entire or partial molecular chain of the ultra-high molecular weight polyethylene can be disentangled, thereby uniform solution can be formed. Therefore, the solubility of ultra-high molecular weight polyethylene particles in the solvent is critical, which determines the production efficiency and property of the fiber product to a large extent.
However, the solubility of ultra-high molecular weight polyethylene becomes poorer and poorer as the molecular weight thereof increases, and thus the production efficiency and the property of the fiber product would become poorer in the spinning process. On the other hand, although when the ultra-high molecular weight polyethylene has low molecular weight the solubility thereof would certainly increase, the tensile strength of the fiber product made therefrom would be far from satisfactory. The ultra-high molecular weight polyethylene in the prior art cannot have both good solubility and high molecular weight, and be spun into fiber with high stretching strength at the same time.